Exercise has well-known beneficial effects on physical and cardiovascular fitness. Less well-known are the effects of exercise on mental health (e.g., reductions in anxiety and depression, and improvements in sleep patterns and cognitive abilities.) Knowing the bases for the mental effects can provide important clues to the biochemical bases of anxiety, depression and sleep disorders. Understanding the neural effects 'of exercise and muscle disuse might also be useful for developing interventions for humans unable to partake in physical activity (e.g., due to paralysis, chronic bed rest or anti-gravity environments). Previous investigations of the neural adaptations to exercise have included reports of increased levels of neurotransmitters, neurotrophic factors and an increase in capillary density. In the latter finding, capillary density increased in forelimb representations of the cerebellum following chronic exercise in adult rats. The data are suggestive that the adult vascular system continues to conform to changing needs for the delivery of oxygen and glucose, needs presumably driven by an increase in neural activity. Cytochrome oxidase is a marker for' oxidative metabolism, and its activity has been shown to correspond to levels of neural activity. Specific aim #1 is designed to map changes in cytochrome oxidase activity in the cerebellum, motor cortex and visual cortex of adult rats after 1 month of voluntary wheel running. It is hypothesized that forelimb and hind limb representations, but no other representations, in cerebellum and motor cortex will have an increase in cytochrome oxidase activity in exercising relative to control animals. Cytochrome oxidase activity in the same areas will also be tested in a muscle disuse paradigm involving suspension of hind limbs to relieve the load of the muscles. Specific aim #2 is designed to map changes in the capillary density in the cerebral cortex. It is hypothesized that the density will change forelimb and hind limb representations in the cerebral cortex during chronic exercise or hind limb unloading. Whether or not such changes are restricted to representations of limbs activated or inactivated by the treatment conditions will be tested by comparing capillary density in the visual cortex of control and exercise animals.